The present invention relates to a method and an apparatus for automatically locating the position of photon events on the sensitive imaging array of a two-dimensional charge-coupled device (CCD) at a high level of precision.
A charged-coupled device comprises a photosensitive imaging array which, when a photon is incident thereon, produces a video signal consisting in a sequence of pulses having individual amplitudes proportional to the photon quantity of energy in each sensitive element of the imaging array. Such a device has been used for example in photon counting cameras for use in ground-based astronomical applications where imaging and spectroscopy of faint astronomical objects are important.
As detector technology advances, photon counting systems are expected to become more important, not only in ground-based applications where spatial and temporal resolution are required (e.g. time-resolved imaging, speckle imaging and spectroscopy, optical interferometry, etc.) but also in spaceborne applications where a high sensitivity to incoming light combined with possible time resolution is important.
New generation of sensitive two-dimensional CCD detectors have been combined with image intensifiers and high speed microprocessors for realizing star tracking devices capable of tracking several stars simultaneously. In such spaceborne applications, it is essential to assure a time resolution as high as that afforded by the smaller CCD devices while at the same time providing a large effective imaging area as offered by a detector system with a larger number of picture elements (or pixels). This requires that the location of the photon events is determined and recorded at a high level of precision, in fact at a sub-pixel level of precision.